Subsequently, during Phase 2, participants will cross over to the other study arm (Pbo to MN-166 or MN-166 to Pbo), stabilize, and complete again 6 laboratory sessions. Days 1-10 of the study include a morphine taper, while each of the two subsequent study phases consist of a 7-8-day medication switch and stabilization phase, followed by 6 laboratory sessions over the next 9-10 days (3 sample sessions and 3 choice sessions). During sample sessions, participants will receive one dose of oxycodone (0, 15, or 30 mg/70 kg, PO) that will be available during the choice session the following day. At least 72 hrs after the previous sample session, the second sample session will be completed, followed by a choice session the next day. And then at least 72 hrs after the second sample session, the third and final sample session will be completed, followed by the final choice session the next day. The analgesic, subjective, performance, and physiological effects of oxycodone will be measured. During the choice session, a drug versus money self-administration paradigm will be employed, and the progressive ratio that is completed for drug and/or money will be measured.